1. Field of The Invention
This invention relates generally to obtaining useful work from the polytropic expansion of working fluids by reversible adiabatic expansion. More particularly it is concerned with methods of restoring such working fluids to their original thermodynamic conditions by isenthalpic compression followed by heating, preferably by the use of ambient energy sources.
2. Description of the Prior Art
Useful work production by heating and vaporization of liquids, including cryogenic liquids under pressure, and thereafter expansion to exhaust vapors as well known. U.S. Pat. Nos. 3,451,342 and 3,987,632 well represent this art. However, some of these systems, particularly those designed to employ ambient heat sources, assume a continuous supply of cryogenic fluid from outside sources and are not concerned with recompressing and condensing their exhaust vapors back to the cryogenic liquid state in order to achieve a "closed" thermodynamic cycle. Those prior art systems, such as the one taught in U.S. Pat. No. 3,287,901 that have addressed the problem of closing a thermodynamic cycle are generally characterized by the placement of expansion turbines, pumps, engines, coils, etc., as well as the cryogenic liquid reservoir itself, all inside large and sometimes elaborately insulated cryogenic chambers.
These cooling chamber arrangements have certain practical, as well as theoretical limitations. Practical limitations exist because the cryogenic environments within the cooling chambers are inherently hostile to mechanical equipment, while any heat created by the equipment's mechanical movement is an anathema to the maintenance of the cryogenic conditions needed in the chamber to condense the exhaust vapors. This dilemma represents the practical aspect of the overall problem associated with the introduction of heat into these cooling chambers. There are important theoretical limitations as well; the most important of which is that a direct isentropic expansion from a vapor state to a state of total condensation implies a nearly perfect conversion of the working fluid's latent heat of vaporization, if any, to an amount of work output which would need to be reemployed to restore the working fluid to its original state, thus producing no net useful work. Such a perfect conversion has proven to be an extremely elusive goal and to the extent it cannot be achieved due to the original entropy of the working fluid at its original high pressure state, there must be provided another distinct step of direct heat removal from such chambers in order for the working fluid to arrive at a state of total condensation. That is to say, the latent heat of vaporization must go to some "heat sink" before total condensation becomes possible. Present practice acknowledges the difficulty of achieving direct isentropic expansion to total liquid by carefully providing heat sinks through which the latent heat of vaporization is removed in order to convert a vapor back to a saturated liquid. However, the removal of the latent heat at some low temperature implies both a lower limit of operation for the thermodynamic cycle and an irrevocable loss of the latent heat of vaporization. This invention differs from both forms of the prior art in that it does not attempt a direct isentropic expansion to a liquid nor a total condensation of the working fluid by means of a heat sink. Rather it is concerned with recompression of the working fluid along a path of isenthalpic compression in order to provide a closed thermodynamic cycle that has been freed of the heat sink limitations of the prior art so that the latent heat of vaporization need not be irrevocably discarded in order to repressure the working fluid.